1. Field of the Invention
The invention relates to a method of deriving a control signal for inserting a background signal into parts of a foreground signal, these parts being fixed by a key color, the control signal assuming a first value when the foreground signal represents a color in the region of the key color, and a second value when the foreground signal represents a different color, and in which the control signal has a transition range between the two values, and to an arrangement for carrying out that method.
2. Description of the Related Art
A method, which is often used in television for mixing different pictures of the corresponding signals is the chromakey method. In this method a foreground signal is used which partly comprises a predetermined color--generally blue--which is replaced by corresponding parts of a background signal. The foreground signal and a background signal are applied to a picture mixer which enables the two signals to be switched in dependence upon a control signal derived from the foreground signal.
In known arrangements of this type, the control signal is a switching signal, whereby a first value of this signal is associated with the key color and a second value of the signal is associated with those colors which deviate from this key color. Since the blue wall used for performing the method does not have a constant color, which is particularly due to non-uniform illumination, the aim is to extend the key color region to a possibly large region in the color plane, but in which the colors in the foreground should not be included. However, the sudden switching between the foreground and the background gives rise to errors, even at the smallest shift with respect to time of the switching instant. Unprogrammed switching to the blue wall is very disturbing at the edges of the foreground. Particularly in the case of fine details, color fringes occur, more specifically in the hairline of a person who is standing in front of a background.
In an improvement of the known method, which is also referred to as soft chromakey, a proportional zone is introduced between the key color region and the foreground colors, and the control signal is no longer limited to the range of "0" and "0". Then there is a soft fading between the foreground signal and the background signal. zone, simple mathematical derivations are used which result in corresponding geometric shapes, such as diamonds, circles or ellipses. As an example for an already proposed method of this type, reference is made to Application DE 40177878 (PHD 90-252). In this method light color fringes still occur, particularly when or where the proportional zone is not close to the colors representing the foreground.